1. Field of the Invention
The present invention relates generally to fluid reaction surfaces, and more specifically to a turbulator configuration within a turbine airfoil.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
A gas turbine engine, such as an aircraft jet engine or an industrial gas turbine engine used for electric power production, include a turbine in which a plurality of stages of stator vanes and rotor blades are staggered along a hot gas flow path to extract mechanical energy and rotate the rotor shaft of the engine. The turbine vanes and blades in the forward stages of the turbine require complex cooling circuitry to provide air cooling for these airfoils in order to withstand the extremely high temperatures from the hot gas flow. It is well known that the engine efficiency can be increased by providing for a higher hot gas flow entering the turbine. However, the turbine airfoils' material properties limit the temperature at which these components can be exposed.
Providing more cooling air through these hot components could allow for a higher temperature of the hot gas flow. However, the cooling air used for the turbine airfoils is typically bled off from the compressor and therefore would be wasted. Reducing the amount of cooling air used from the compressor would also increase the efficiency of the engine. Thus, it is beneficial to provide for the turbine airfoil hot components to use a minimal amount of cooling air while providing for a maximum amount of cooling in order to maximize the efficiency of the engine.
A cooled turbine airfoil generally includes a plurality of passages formed within the airfoil in which the cooling air flows. Heat transfer from the hot airfoils walls to the cooling air is accomplished by convection and impingement cooling. In order to increase the heat transfer coefficient within the airfoil passage, turbulators ribs have been used to enhance the heat transfer coefficient along the inner surface of the airfoil passage by tripping or disrupting the cooling air boundary layer which is caused to separate from the internal surface and then reattach downstream from the rib. The heat transfer coefficient enhancement is conventionally defined as the convective heat transfer coefficient affected by the ribs divided by the convective heat transfer coefficient over a smooth surface without turbulator ribs, and has values ranging up to several times that of the latter.
Enhancement is conventionally related to the height or projection of the ribs into the internal passage, the distance between opposing walls of the internal passage, and the distance or spacing longitudinally between the ribs. The typical ratio of longitudinal spacing between turbulator ribs relative to rib height ranges from about 5.0 to about 10.0, and the ratio of the rib height to opposing wall distance has values of about 0.07 and up. And, exemplary turbulator ribs may include ribs disposed perpendicularly to the direction of cooling flow, ribs inclined relative to the direction of the cooling airflow, and ribs disposed on opposite walls of the internal passage that are longitudinally positioned either in-line or staggered with respect to each other.
Turbulator ribs provide localized increases in enhancement which decrease rapidly in value downstream from each individual rib. Accordingly, the ribs are typically uniform in configuration, uniform in height or projection into the internal passage, and uniform in longitudinal spacing there-between for obtaining a generally uniform, or average, enhancement along the surface of the blade cooled by the ribs.
The various conventional turbulator ribs result in different amounts of enhancement, along with pressure losses associated therewith. Since the ribs project into the internal passage and partially obstruct the free flow of the cooling air there-through, they provide resistance to the flow of the cooling air which results in pressure losses. Although higher ribs generally increase enhancement, the pressure drop associated therewith also increases, which, therefore, typically requires an increase in supply pressure of the cooling airflow to offset the pressure losses. Accordingly, the effectiveness of turbulator ribs must be evaluated by their ability to provide effective enhancement without undesirable levels of pressure losses associated therewith.
The cooling passages within an airfoil typically have circular, rectangular, square or oblong transverse cross-sectional shapes. In a rotating turbine blade having a serpentine flow cooling circuit including longitudinally-oriented cooling passages of square cross-sectional shape, Coriolis (rotational) forces will increase the heat transfer coefficient along certain walls of the passage and decrease the heat transfer coefficient along other walls of the passage as compared to a non-rotating airfoil such as a stator vane. The Coriolis force compresses the cooling air against one side of the square passage, resulting in an increase in the heat transfer coefficient at that side while decreasing the heat transfer coefficient at the opposite side. This creates an uneven transverse cross section blade temperature profile which creates hot areas that must be compensated for by increasing the cooling air flow. Increasing the cooling air flow by bleeding off more compressed air from the compressor would decrease the engine efficiency.
FIG. 1 shows a cut-away view of a prior art convectively cooled turbine blade for a gas turbine engine. Turbulators are arranged along the three radial passages extending from the blade root to the blade tip. FIG. 2 shows a cross sectional view of the blade taken along the line shown in FIG. 1 with the three radial passages having turbulators formed along the pressure side wall and suction side wall of the passages. FIG. 3 shows a cross sectional view taken along the line shown in FIG. 1. this prior art cooling design scheme comprises conventional channel flow cooling that is augmented with long, skewed turbulators that are used for the blade leading edge, mid-chord section, and trailing edge. As the cooling air flows through the skewed turbulator, the leading edge of the turbulator (as indicated in FIGS. 3 and 4) trips the thermal boundary layer of the cooling fluid and subsequently augments the local heat transfer coefficient and thus enhances airfoil local cooling performance. As a result of this boundary layer tripping, vortices are generated and propagated along the turbulator from the leading edge to the trailing edge of the turbulator. As the vortices propagate along the full length of the turbulator, the boundary layer becomes progressively more distributed or thickened, and consequently the tripping of the boundary layer becomes progressively less effective. The net result of this boundary layer growth is significantly reduced heat transfer augmentation.
An alternative turbulator arrangement in the prior art is the Chevron formation shown in FIG. 5 which creates double leading edges for the turbulence promoter to increase the channel heat transfer enhancement. However, with this type of turbulence promoter arrangement, two thick boundary layers are generated at the junction of the turbulence promoters in the middle of the flow channel. The interaction of both vortices can be eliminated by segmenting the Chevron turbulator into two separated skew turbulators as represented in FIG. 6. The small opening between the two skewed turbulators allow for a small amount of the cooling air to blow through the slot and break up the vortices. Regardless of the turbulator arrangement, the thick boundary layer built-up by the vortices still reduce the tripping effect by the incoming cooling flow and thus reduce the heat transfer augmentation along the turbulator.
It is therefore an object of the present invention to provide for a turbine blade having an internal cooling air passage that has an improved heat transfer coefficient with reduced cooling air flow than those in the cited prior art references.
It is another object of the present invention to provide for a turbine blade with improved turbulator ribs for increasing the heat transfer coefficient.
It is another object of the present invention to provide for a turbulator rib that provides for an improved heat transfer coefficient that those of the cited prior art references.